Shedding Issues!?

I have a 3 yr old, Bull Mastiff, that sheds constantly. After brushing, it is like you didn't even touch her. I read that this breed doesn't shed that much. Anything with hair is going to shed, but this is all the time. I've changed her food, shampoo, and asked the vet. No dice. She isn't going bald or anything like that, but is this really normal for this breed?? Please help.